An image forming apparatus of an electrophotographic printing type such as a printer includes a fixing device for thermally melting a toner image formed on a recording material (sheet of recording paper, recording sheet) so as to fix the toner image to the recording material. As an example of fixing methods carried out in this fixing device, there is known a method of fixing a toner image on a recording material in a non-contact manner by irradiating the toner image with light. Since the toner image is heated in a non-contact manner, no warming up is needed in this method unlike in a roller fixing method that is a conventional contact heating method.
As a fixing device making use of light irradiation as described above, a laser fixing device as disclosed in Patent Literature 1 has been proposed in which a toner image is fixed by utilizing laser power. Patent Literature 1 discloses provision of an optical unit which has first and second laser light scanning irradiation means and forms a latent image by means of the first laser light scanning irradiation means, and of means for fixing a toner image, which is developed on a recording material, by means of the second laser light scanning irradiation means using light energy as a heat source. The laser light irradiated from the second laser light scanning irradiation means is converted into parallel light by means of a collimating lens constituted by spherical and/or aspherical surfaces, and then focused to a position through which a predetermined recording material passes, by means of f-θ lenses via a reflecting mirror for fixation. The fixation is carried out at this focal position. In this way, the position at which the fixation is carried out in the process of carrying the recording material is determined by adjusting to the focal distance.